Jade Thirlwall was left fighting in hospital after a battle with anorexia.
The Little Mix star started dramatically losing weight when she lost a close relative but didn't understand how badly she needed help.
Sharing her story in the group's book Our World, she said: "My periods stopped and things were getting out of control but I don't think I really cared about what was happening to me. I felt so depressed at the time that I just wanted to waste away and disappear ...
"Anorexia is a self-destructive thing and you become stubborn, so when people are trying to tell you something you get it into your head that they're against you and you're not going to listen.
"It took going to hospital to make me realise that it wasn't a game, it was something really serious. They sat me down in the clinic and were quite tough at first, spelling it out, 'You're destroying your body and if you keep doing this you will die.'"
Meanwhile, Jade and her bandmates - Perrie Edwards, Leigh-Anne Pinnock and Jesy Nelson - previously opened up about their close bond, insisting they are more like sisters than friends.
Leigh-Anne shared: "A force to be reckoned with... for 5 years my girls and I have stuck together, been there for each other and grown into not only best friends but sisters.. that will always have each other's backs! We've been through break ups, we've been at our lowest but we have always helped each other and come out stronger on the other end! Call me the luckiest girl alive, because this is a dream I am living, but I get to live it with 3 of the most talented, special individuals alive. Now let's smash the s**t out of this #shoutoutomyex. (sic)"